Atrial fibrillation is associated with formation of emboli which can cause thrombi, strokes, heart attacks, kidney infarcts and pulmonary embolisms. Once atrial fibrillation is initiated, it is often difficult to reverse. An atrium which is in fibrillation does not efficiently pump blood, and as such, the atrium may dilate. Such dilation perpetuates a vicious cycle because the occurrence of atrial fibrillation in a dilated heart is more difficult to reverse than in a normal heart, since a dilated atrium increases the conductive pathway length which further increases the time required to complete one cycle of atrial contraction. A longer, conductive pathway increases the probability of facilitating undesirable conduction circus motions in an atrium.
There have been a number of approaches to treating atrial fibrillation. One approach is a pharmacological treatment. While such treatment can reduce the tendency of an atrium to undergo fibrillation, once such atrial fibrillation begins, however, the pharmacological treatment to stop atrial fibrillation is not practical as it is difficult for a patient to administer a pharmacological agent to stop ongoing atrial fibrillation. The fact that the pharmacological treatment is not automatic and instantaneous makes such treatment of an ongoing atrial fibrillation difficult, if not impossible.
Another method of treating atrial fibrillation is to ablate (burn) portions or patterns in the heart to stop the circus motion that is associated with atrial fibrillation. For example, circumferential patterns may be burned around the four left pulmonary veins. Disadvantageously, this method is very surgically invasive and often provides only marginal efficacy.
Still another approach is a device-based approach using a stimulator that delivers pulses through a single electrode attached to an atrium. One such method involves over-pacing the atrial contraction rate to disrupt the normal atrial rhythms with the intent to shorten the long, diastolic periods and to cancel the premature atrial contractions found in a fibrillating atrium. While mildly successful, use of over-pacing in a single-electrode based system can cause pacemaker-mediated, congestive heart failure.
Another device-based method uses defibrillators to shock the atrium in order to stop atrial fibrillation. The high levels of energy used to produce the shocks, however, can cause pain and otherwise be disconcerting to a patient. Methods have been described which use lower, less discernable energy levels, as described in U.S. Pat. No. 5,620,468 issued to Mongeon, et. al., which patent is incorporated herein by reference in its entirety. Other patents or publications which are relevant to the treatment of atrial fibrillation (all cited in the '468 patent) include: U.S. Pat. No. 3,937,226; PCT No. U.S. 92/02829 (Publication No. WO 92/18198); U.S. Pat. No. 5,356,425; and U.S. Pat. No. 5,334,221.
It is apparent that what is needed is an improved device-based therapy that can suppress the onset of atrial fibrillation and also treat atrial fibrillation once it is detected.